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Hawaiian Islands Synoptic Discussion and Guidance

941
FXHW60 PHFO 201356
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
356 AM HST Sat Jan 20 2018

.SYNOPSIS...
High pressure across the Eastern Pacific and a trough in the
Central Pacific will lead to a weak ridge north of the Hawaiian
Islands through the weekend. Expect easterly trade winds for most
islands with modified afternoon sea breezes along southern and
western slopes of Kauai and Oahu through Sunday. Unsettled weather
continues over the state next week, ahead of a stronger frontal
system moving across the Central Pacific. Easterly trade winds
will continue through Wednesday. Wind speeds will weaken and
directions will veer towards the southeast by Wednesday night.
Land and sea breezes will become more widespread on Thursday and
Friday. Vog may move northward into the smaller islands by the end
of next week.

&&

.DISCUSSION...
The satellite imagery over the state shows a large band of
unstable clouds drifting across the Hawaiian Islands from east to
west. An upper level trough shown in water vapor imagery over
Kauai will help enhance shower activity over Kauai and Oahu later
today. A subtropical jet stream lingers just south of the Big
Island.

The large scale picture remains the same with a strong high
pressure system drifting away from the state into the East
Pacific and a cold frontal system moving across the Central
Pacific. The positions of these two large scale systems will keep
a weakened ridge north of the islands along with easterly trade
winds from Molokai to the Big Island. For Kauai and Oahu, a
modified easterly trade wind regime will develop through Sunday
with periods of southeasterly winds across portions of the
southern slopes and on-shore winds over western slopes each
afternoon. The forward motion of the cold front will stall roughly
200 miles northwest of Kauai by Sunday evening.

Monday and Tuesday...A high pressure ridge between two low
pressure systems will produce moderate easterly winds each day
across all islands. Expect a wet weather pattern with the highest
rainfall chances over the windward slopes of the Big Island and
Maui as unstable clouds upstream move into the eastern slopes of
each island.

Wednesday through Friday...A stronger low pressure system will
move into the Central Pacific on Wednesday with a trailing cold
front approaching the islands from the northwest. Easterly winds
will weaken and directions will veer towards the southeast by
Wednesday night as the front gets closer. Land and sea breezes
will become more widespread on Thursday and Friday. Prefrontal
convergence bands will drive precipitation chances along with
daytime heating and convection over island interior sections
producing afternoon clouds and scattered showers. VOG from the
volcano on the Big Island may also be drawn up with the southeasterly
wind flow over the smaller islands on Thursday and Friday.

Slight changes were made to the forecast grids this morning.
Easterly wind speeds were increased on Monday and Tuesday, and
higher precipitation chances and clouds were added for next weeks
unsettled weather conditions.

&&

.AVIATION...
Easterly trade winds will ease slightly today, particularly over
the western islands, as a trough of low pressure approaches from
the west. Clouds and showers will continue to focus primarily
over windward and mauka areas through the period, with occasional
MVFR cigs/vsbys in SHRA expected.

AIRMET Sierra is in effect for mountain obscuration across
windward portions of Kauai, Oahu, Molokai, and Maui. Conditions
may improve in some areas later this morning.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands. Some improvement is
possible over the western end of the state later this morning.

&&

.MARINE...
Moderate to fresh trade winds will continue with a surface high
located northeast of the state. Trade wind swell generated to the east
will keep seas above 10 feet, and the Small Craft Advisory has
been extended through Sunday for most zones. The High Surf
Advisory (HSA) for east facing shores will continue through
Sunday.

Surf will remain below advisory levels for the remaining shores
through this weekend. A small northwest swell (330 degrees) is
expected arrive tonight, with a reinforcing long period, west-
northwest (300 degrees) swell expected Sunday night. The WNW swell
will peak Monday night and gradually declining through Wednesday.
This second swell may warrant an HSA for exposed west and north
facing shores Monday and Tuesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Sunday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters-
Kaiwi Channel-Maui County Windward Waters-Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Windward Waters-Big Island
Leeward Waters-Big Island Southeast Waters.

&&

$$

Discussion...Bohlin
Aviation...Jelsema
Marine...Morrison

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office